GdkPixbufFormat needs a registered GType, otherwise it will not be possible to use it cleanly from language bindings.

Something like this should work:

static GdkPixbufFormat*
gdk_pixbuf_format_copy (const GdkPixbufFormat *format)
{
  GdkPixbufFormat *new_format;

  g_return_val_if_fail (format != NULL, NULL);

  new_format = g_slice_new (GdkPixbufFormat);
  *new_format = *format;
  return new_format;
}

static void
gdk_pixbuf_format_free (GdkPixbufFormat *format)
{
  g_return_if_fail (format != NULL);

  g_slice_free (GdkPixbufFormat, format);
}

GType
gdk_pixbuf_format_get_type (void)
{
  static GType our_type = 0;

  if (our_type == 0)
    our_type = g_boxed_type_register_static (g_intern_static_string "GdkPixbufFormat"),
                         (GBoxedCopyFunc)gdk_pixbuf_format_copy,
                         (GBoxedFreeFunc)gdk_pixbuf_format_free);
  return our_type;
}